I don't think there are any more civil JT3C-powered aircraft flying, though a very few such Boeing 720s are maybe stored. I think NASA might still have a KC-135 with the military version of the enginge (J57). As for the JT4A, it's finally pretty much had it, too. There may be 1 or so DC-8s with it in storage, but all the 707s still with it (a few with the Israeli air force) appear to be retired. General Electric's 707-321 that once had JT4s is shown partly dismantled on a recent airliners.net 707 section photo. But you can still see the Conway on the RAF's 20 or so VC-10s (no more on 707s and DC-8s), the Spey on BAC-111s and I think also on RAF Nimrods, and the JT3D (turbofan derivative of JT3C) on many extant and airworthy DC-8s and 707s.

There are quite a few of F28s still flying here...
Merpati has quite a number, about 20 - 30 of them with only 1/2 flying
Pelita has about half a dozen.
Transwisata has recently added F28 to its fleet... 2 of them.
Batavia tookover Bouraq/Bali Air's F28...
Garuda's Citilink Division, is retiring their F28s after last month's incident. This will be the 2nd time the F28 is retired from Garuda.

This plane is quite irreplaceable

Now, I better not start counting the RR Darts on HS748s and F27s in my country!

I just requested a owner of a 720 based at PHX that I saw it last week when I landed there. Also at PHX there are an ANG base with KC135E's and like EdT said America West still flies the 737-200 ( I actually saw 2 of them still flying... ). The other ones, well I know that GE had an old 707-321 stored somewhere in the desert but I don't know if is still in flying condition.
Regards